Thursday, September 15, 2016

14 comments:

I don't think that there is any reasonable doubt about the modern anthropogenic climate change, but I wouldn't put too much faith in the xkcd cartoon, which compares millennial scale reconstructions with modern measurements at a much finer time scale. There is a certain amount of evidence that mid Holocene climate was as warm or warmer than current temperatures. See, e.g., http://www.ncdc.noaa.gov/paleo/globalwarming/holocene.html

We also know that that warm climate seems to ave been driven by orbital forcings that are not as large today.

"We also know that that warm climate seems to have been driven by orbital forcings that are not as large today."

As Randall actually implies with the two captions at 18500 and 4750:

"Earth begins to cool slowly, mainly due to regular cycles in its orbit."

No doubt he could have worded them and others much better, but this is a work of popular communication, not a climate science primer. All the minor nitpicking I've seen prompts me to recall the maxim: Don't let perfect be the enemy of good.

CIP #1 — from your link it appears the mid-holocene was a northern hemisphere phenomenon and that it was warmer in the summer but cooler in the winter. It's not clear to me whether the average is warmer or cooler than today.

The irritating bit is that the author refers ('skeptically') to computer models in describing the reconstructions. It seems to me there's a difference between using proxies like isotope analyses and tree rings and computer models. But the notion that there's a special 'license to be skeptical' when computer models are involved is irritating. Reliability (as in 'fit for purpose') is the issue, and we have good reason to think many kinds of computational models are reliable. I suspect over-emphasis on 'truth' is part of the problem- models aren't the truth (neither is any measurement we make) but both are often reliable.

Bob Covey - Our current warming is also more pronounced in the Northern Hemisphere but winter has warmed more than summer, so the signature is opposite to that of the mid Holocene warming. There is a certain amount of evidence for significant glacial melting in Europe during the mid Holocene and later - probably due to the warmer summers.

I mention this mainly because the more informed skeptics are full of this sort of partial information, and quick to confound our current warming with those earlier ones, even though some of them are well understood from basic principles and have different seasonal signatures.

Beach ridges and driftwood *do* indicate there was more open water around the Greenland coast, but given the other evidence available this shouldn't be taken as evidence that the arctic ocean as a whole had more open water than today.

Prof. Sven Funder, who has done much work in the beach ridge and driftwood areas, originally thought these indicated lower extent during the HCO (He has a quote or two that are favorites among the denialistas). He later became agnostic on the subject when he saw data from other geographical areas around the arctic.

Our results indicate that global mean temperature for the decade 2000–2009 has not yet exceeded the warmest temperatures of the early Holocene (5000 to 10,000 yr B.P.). These temperatures are, however, warmer than 82% of the Holocene distribution as represented by the Standard 5×5 stack, or 72% after making plausible corrections for inherent smoothing of the high frequencies in the stack (Fig. 3). In contrast, the decadal mean global temperature of the early 20th century (1900–1909) was cooler than >95% of the Holocene distribution under both the Standard 5×5 and high-frequency corrected scenarios. Global temperature, therefore, has risen from near the coldest to the warmest levels of the Holocene within the past century, reversing the long-term cooling trend that began ~5000 yr B.P.

Climate models project that temperatures are likely to exceed the full distribution of Holocene warmth by 2100 for all versions of the temperature stack (Fig. 3), regardless of the greenhouse gas emission scenario considered (excluding the year 2000 constant composition scenario, which has already been exceeded). By 2100, global average temperatures will probably be 5 to 12 standard deviations above the Holocene temperature mean for the A1B scenario based on our Standard 5×5 plus high-frequency addition stack (Fig. 3).

CIP, that's why I'm skeptical. Impact modeling requires the ability to model at small scales. Let's be honest, you want to make sure a giant cyclone isn't going to strike New Mexico, and Eli needs to figure out what's going to happen to California's wine. I'm more focused on showfall over Greenland, etc.

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Eli Rabett

Eli Rabett, a not quite failed professorial techno-bunny who finally handed in the keys and retired from his wanna be research university. The students continue to be naive but great people and the administrators continue to vary day-to-day between homicidal and delusional without Eli's help. Eli notices from recent political developments that this behavior is not limited to administrators. His colleagues retain their curious inability to see the holes that they dig for themselves. Prof. Rabett is thankful that they, or at least some of them occasionally heeded his pointing out the implications of the various enthusiasms that rattle around the department and school. Ms. Rabett is thankful that Prof. Rabett occasionally heeds her pointing out that he is nuts.